The invention relates to a tool for the machining of bores in workpieces and to a method of machining bores with such a tool.
During about the last hundred years, production technology has been brought to its current state by intensive and worldwide development. In modern automobile plants, for example, four-cylinder mass-production engine blocks are produced with cycle times less than one minute. Machining is carried out in transfer lines. They comprise a large number of machining stations lined up in a row behind one another (turning, boring, milling machines and the like), which are permanently linked together in space and time by an automatic workpiece transport device. The work progresses under cyclic control. As soon as all the machining units have fulfilled their respective task, the workpieces are transported onward by one cycle distance to the next machining station.
This procedure permits only low flexibility, that is to say adaptability to other machining tasks. In addition, high investment costs are needed for the transfer line, for which reason such plants can be used economically only in mass production and large-scale production. Model changes, design changes resulting from technical improvements or expansions to the product range are barely possible, or possible only to a restricted extent with a high outlay, mostly only by utilizing the yearly plant holidays.
At the same time, a changing consumer behavior with more frequent model changes and, in many cases, little guaranteed statements about the product politics to be expected in the future and to be included early in the calculation plans, as well as the displacement of production, caused by cost pressures, away from the classical industrial countries demand a more flexible production method with smaller batch sizes and the lowest possible overall costs.
In principle, this problem is solved by the use of flexible production systems. They satisfy the contradictory aims of higher productivity and flexibility. A number of NC machines which supplement one another or replace one another, generally machining centers, are linked to one another by a computer-controlled workpiece transport device.
In the production of cylinder bores in engine blocks, a high quality must be achieved, that is to say it is necessary to work to very small dimensional, positional, shape and surface tolerances. The desired nominal dimension of the cylinder bore is as a rule not identical to the nominal dimension of the cutting edge on the tool, since the wear-dependent cutting force gives rise to deformations on tool and workpiece, the interface between machine and tool exhibits geometric deviations, and the accuracy of reproduction of the tool holder is limited. Since all the materials have a wearing effect to a greater or lesser extent on the cutting edge which defines the dimension, the actual diameter of the bores becomes smaller and smaller with the number of workpieces machined. Because of that, the precision boring cutter has to be readjusted very much earlier, generally for reasons concerned with statistical quality control, at the latest when the lower dimensional tolerance is reached.
For the purpose of precision adjustment and cutter wear compensation, control devices for the cutter are provided. Therefore, on the tool the setting by means of elastic deformation of simple or parallelogram bending beams or diaphragms is provided as an articulated connection between the tools or tool adapters, which can mostly be pivoted, and the machine spindle.
It is also known to perform the setting by means of linear slide guides of all types having tool holder slides which can be adjusted essentially in the radial direction, for example so-called planar rotary heads, or by means of single-armed or two-armed levers or rockers or links as cutter holders.
The elastic systems have the disadvantage of a very limited adjustment range, different control forces and therefore nonlinear influences, as well as only moderately stiff, vibration-sensitive control elements.
The control devices with slide guides take up a great deal of space with large external diameters, high weight and moment of inertia and problems in balancing. Slide, slide guide and control element must run without play or will have running play and backlash and generally also have to be clamped firmly after the control operation.
The drive to the control device is carried out by means of controlled actuation of a control rod in the drive spindle, by means of hydraulic or pneumatic control pistons, by means of internal coolant supply, hydraulic oil supply or by compressed air supply.
The object of the invention is to develop the generic tool and the generic method in such a way that the tool can be used both on standard machines and on special machines, preferably NC-controlled machines and is distinguished by a favorable price/performance ratio, by high accuracy, by stiffness and operational reliability and by a compact construction.
This object is achieved by the tool and method according to the present invention.
In the tool according to the invention, the radial adjustment of the cutter is initiated by the centrifugal force-actuated element. It is therefore possible for the cutter holder to be adjusted very simply via the rotational speed of the tool, and therefore for the cutter seated on said holder to be readjusted. The tool according to the invention is distinguished by a construction of simple design, so that it can be produced cost-effectively. It can be used both on special machines and on standard machines. For example, using the tool according to the invention, which is driven so as to rotate about its axis, housings in machine and vehicle construction, especially multicylinder engine blocks and cylinder heads for internal combustion engines, compressors, injection pumps and so on can be machined very accurately. In the method according to the invention, cylinder bores can be produced with high accuracy under track control in at least two, preferably in three, NC axes. As a result of the design according to the invention, highly accurate readjustment of the cutter is possible, this readjustment lying in the xcexc range. It is therefore possible to compensate for wear of the cutter simply and nevertheless highly accurately. The cutter provided on the cutter holder is advantageously a cutter for final machining, so that after the final machining, the tool has a high machining accuracy because of the design of the tool according to the invention. By arranging further cutters on the same tool body, it is also possible for additional machining operations, for example chamfering, to be carried out, or for the production operation to be divided up into coarse and fine machining.